1. Field of the Invention
This application relates generally to systems and methods for the detection and quantification of gas hydrates. More specifically, this application relates to systems and methods for the detection and quantification of gas hydrate concentration and distribution from seismic data. Even more specifically, this application relates to methods, apparatus and computer usable program code for the detection and quantification of gas hydrate concentration and distribution from seismic data utilizing multiple seismic attributes in a multivariable analysis.
2. Background of the Invention
Gas hydrates are crystalline solids consisting of a gas molecules surrounded by water molecules. The structure is similar to ice, except that the crystalline structure is stabilized by the presence of the gas molecule. The two molecules are not chemically bonded, but mechanically intermingled without true chemical bonding. Gas hydrates may be formed from a number of gasses having an appropriate molecular size, including carbon dioxide, hydrogen sulfide and several low-carbon-number hydrocarbons, including methane. Natural gas hydrates are modified ice structures enclosing methane and possibly other natural gas molecules.
Gas hydrates tend to form in the pore spaces of sediment layers beneath the ocean floor. However, these hydrates may also be seen as modules or deposits of pure hydrate. Gas hydrates are stable at low temperatures and high pressures typically found in the shallow section of a deep water region, such as at water depths greater than about 500 meters. They occur in shallow sediments beneath the sea floor. The actual depth at which gas hydrates are stable may vary depending on the specific conditions at a location. Gas hydrates may also be stable in association with permafrost, both on- and off-shore. Natural gas hydrates act as a gas concentrator. For example, one unit volume of hydrate is equivalent to about 164 unit volumes of methane gas at standard conditions. Often, however, the hydrate itself is dilute in the sediment, occupying a few percents of the volume on average.
Locating likely areas of concentrated gas hydrates has therefore become an interest to those looking for alternative fuel sources. Remote seismic sensing methods have been proven useful in detecting and characterizing gas hydrates. The presence of a BSR may facilitate the detection of gas hydrates. A BSR is a high-amplitude reflector that approximately parallels the seafloor, and which results from the strong acoustic impedance contrast between the gas hydrate-bearing sediments above the reflector and the underlying sediments containing free gas. Because the BSR follows a thermobaric surface rather than a structural or stratigraphic interface, it is normally observed to crosscut other reflectors in conventional seismic section. It is also known that the existence of a BSR does not guarantee the presence of gas hydrates above it, nor does it facilitate the quantification of hydrates.
Locating gas hydrates is relatively easier than determining the relative concentrations of the gas hydrates within the subsurface rock sediment. Mathematical modeling of the ocean floor, using drill data, as well as seismic data, has improved gas hydrate detection and sensitivity. However, anomalies between results obtained using compression seismic waves (p-waves) and shear seismic waves (s-waves) individually, often yields differing results, injecting uncertainty in the detection of the gas hydrates.